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S. Ackemrann et alii, Frattura ed Integrità Strutturale, 34 (2015) 580-589; DOI: 10.3221/IGF-ESIS.34.64 587 observed for the major cracks (Fig. 7a and 7b) as well as for minor cracks (Fig. 7c and 7d) under both straining conditions. The investigated minor cracks had crack lengths of maximum 0.2 mm after equibiaxial cycling and maximum 0.7 mm after shear cycling. They extended mostly over several grains and crack linkage was observed as well. Minor cracks after equibiaxial loading showed different directions (Fig. 7d), whereas most investigated cracks after shear loading were oriented parallel to the loading axes A and B (Fig. 7c). These cases correspond to mode I crack directions. However, a few cracks with 45° orientation to the loading axes which corresponds to mode II were observed in the shear specimen as well (Fig. 7c). The mode II orientation was found for crack lengths between 0.05 and 0.13 mm, thereafter these cracks branched into two mode I cracks. Locally, cracks propagated along deformation bands. Crack initiation was mostly found on impurities, but also in the matrix along deformation bands (e.g. Fig. 7c). Figure 6 : Surface cracks after biaxial fatigue of steel PM 16-7-6 with strain ratios of a)  = 1 (equibiaxial loading), b)  = -1 (shear loading) and c)  = 0.5 at Δ  vM /2 = 0.4 · 10 -2 (a, b, c) as well as d)  = -1 at Δ  vM /2 = 0.5 · 10 -2 , obtained by electron beam monitoring of the gauge area indicated by circles, loading axes A and B marked.  Figure 7 : Micrographs of surface cracks after biaxial cycling with  = -1 (a, c) and  = 1 (b, d) at Δ  vM /2 = 0.4 · 10 -2 of TRIP steel PM 16-7-6. Major cracks (a, b) and cracks smaller than 0.1 mm (c, d) show crack branching. S UMMARY he cyclic deformation and fatigue behavior of a metastable austenitic TRIP steel was investigated under uniaxial and different biaxial-planar loading conditions and discussed by using observations of surface cracks. Shear fatigue caused both the highest number of cycles to failure and the highest  ’-martensite volume fractions, whereas loading with  = -0.5, -0.1 and 0.5 resulted in the lowest fatigue lives as well as the lowest  ’-martensite contents. T